The hemolytic efficiency of the genetic variants of the third component of complement (C3), native C3 in serum and highly purified C3 were estimated. The specific hemolytic activities of each of the partially purified variants were similar to that of C3 in whole serum but only about 20% that of highly purified C3. In the highly purified preparation, approximately 300 C3 molecules were bound for each hemolytic site.